1. Field of the Invention
This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2007-068879, filed on Mar. 16, 2007, the disclosure of which is incorporated herein in its entirety by reference.
The present invention relates to a mobile radio communications system and, more particularly, to a control method and device of resource allocation among control signals and reference signals (pilot signals).
2. Description of the Related Art
In Long Term Evolution (LTE), which is currently being standardized by the 3rd Generation Partnership Project (3GPP), single-carrier transmission is employed as an uplink access scheme in broadband radio access. The single-carrier transmission is an access scheme excellent in power efficiency because the peak-to-average power ratio (PAPR) can be suppressed low in comparison with multi-carrier transmission such as orthogonal frequency division multiplexing (OFDM). Therefore, considering that a mobile station, which is also called user equipment (UE), has limited battery capacity, the single-carrier transmission can be said to be an access scheme suitable for uplink. Hereinafter, a technology related to the present invention will be described, taking an LTE-based radio communications system as an example.
FIG. 1A is a block diagram showing a radio mobile communications system in general, and FIG. 1B is a format diagram showing a signal frame structure in LTE. As shown in FIG. 1B, according to LTE, one slot is equivalent to 0.5 msec and is composed of seven long blocks (LBs), and two slots constitute one transmission time interval (TTI). Incidentally, the TTI is as long a time interval as a plurality of blocks (a transport block set) transferred at a time between the physical layer and the MAC layer. In this figure, the bandwidth (the number of subcarriers) of a single frequency resource block (RB) is 12 subcarriers, and control signals and/or reference signals of individual mobile stations (UEs) are placed in each frequency resource block. In FIG. 1B, a resource block is composed of 7 LBs in time direction and 12 subcarriers in frequency direction.
An uplink control signal needs to carry a data-non-associated control signal, which is feedback information about a downlink signal. The data-non-associated control signal includes at least “Acknowledgment/Negative Acknowledgment” (hereinafter, referred to as “ACK/NACK”), which indicates whether or not downlink information has been received without error, as well as channel quality indicator information (hereinafter, referred as “CQI”), which indicates the quality of the downlink channel, and the like. Hereinafter, this data-non-associated control signal will be simply referred to as “control signal.”
In such a control signal, it is preferable that the transmission delay of ACK/NACK be short. As for CQI, the transmission frequency is determined with consideration given to the transmission overhead and the moving speed of the mobile station. Accordingly, there are some occasions when the frequency of transmission of ACK/NACK differs from that of CQI, in which case mobile stations transmitting three types of control signals coexist within a single TTI: those which transmit ACK/NACK only, those which transmit CQI only, and those which transmit both of ACK/NACK and CQI. In addition, since the amount of ACK/NACK information is smaller than that of CQI information, these three types of control signals differ from each other in amount of information and transmission bandwidth.
In LTE, it is being considered that frequency division multiplexing (FDM) and/or code division multiplexing (CDM) are used to multiplex users with respect to uplink control signals (see 3GPP TSG-RAN WG1 #47 R1-063448, Qualcomm Europe, “Structure and Link Analysis UL Control Signaling,” November 2006). In the case of CDM, CAZAC (Constant Amplitude Zero Auto-Correlation) sequence is predominant to be used for spread signals. For example, as one example of CAZAC sequence, Zadoff-Chu sequence represented by the following equation 1 has been known (see B. M. Popovic, “Generalized Chirp-Like Polyphase Sequences with Optimum Correlation Properties,” IEEE Transactions on Information Theory, Vol. 38, No. 4, PP. 1406-1409, July 1992):
                                          c            k                    ⁡                      (            n            )                          =                  {                                                                      exp                  ⁡                                      [                                                                                            j2π                          ⁢                                                                                                          ⁢                          k                                                L                                            ⁢                                              (                                                                                                            n                              2                                                        2                                                    +                          n                                                )                                                              ]                                                                                                where                  ⁢                                                                          ⁢                  L                  ⁢                                                                          ⁢                  is                  ⁢                                                                          ⁢                  even                                                                                                      exp                  ⁡                                      [                                                                                            j2π                          ⁢                                                                                                          ⁢                          k                                                L                                            ⁢                                              (                                                                              n                            ⁢                                                                                                                  ⁢                                                                                          n                                +                                1                                                            2                                                                                +                          n                                                )                                                              ]                                                                                                where                  ⁢                                                                          ⁢                  L                  ⁢                                                                          ⁢                  is                  ⁢                                                                          ⁢                  odd                                                                                        (                  Equation          ⁢                                          ⁢          1                )            where n=0, 1, . . . , and (L−1), L is the length of a sequence, and k is a sequence number which is an integer prime to L.
CAZAC sequences are sequences which have constant amplitude in both of the time and frequency domains and produce an autocorrelation value of zero at a phase difference of any value other than zero. With the CAZAC sequences, because of their property of perfect autocorrelation, signal separation is possible without interference from other users in the case of CDM. Moreover, the CAZAC sequences are suitable for use in channel estimation in the frequency domain because of the constant amplitude also in the frequency domain. Accordingly, a CAZAC sequence is used as a reference signal (pilot signal) sequence.
FIG. 2 is a diagram showing an example of the allocation of resources to a control signal ACK/NACK and/or a control signal CQI in the frame format in LTE. As mentioned above, according to LTE, one slot is equivalent to 0.5 msec and is composed of seven long blocks (LBs), and two slots constitute one transmission time interval (TTI). In this example, where the control signals of individual mobile stations (UEs) are code-division-multiplexed and CAZAC sequences are used as spreading codes, only those mobile stations which transmit signals using the same transmission bandwidth can be orthogonally multiplexed. Additionally, it is assumed that every mobile station performs single-carrier transmission, with no application of multi-code transmission or multi-carrier transmission, by which the PAPR is increased.
In this case, to achieve the orthogonality between the control signals of mobile stations, it is necessary to time-division-multiplex ACK/NACK and CQI at LB level. In FIG. 2, shown is an example where, within a TTI, resources are allocated to ACK/NACK only (except a reference signal) for a mobile station UE1, resources are allocated to CQI only (except a reference signal) for a mobile station UE2, and resources are allocated to ACK/NACK and CQI (except a reference signal) for a mobile station UE3. The reference signals are used to demodulate control signals.
However, problems arise that required conditions are not satisfied and that the efficiency of resource usage declines, if resources are allocated in a fixed amount to a plurality of control signals which differ from each other in amount of information, transmission frequency, required condition of quality, and the like, such as the above-mentioned three types of control signals: ACK/NACK only, CQI only, and ACK/NACK and CQI (hereinafter, also referred to as “ACK/NACK & CQI” where appropriate).
For example, referring again to FIG. 2, the control signals of the mobile station UE3, ACK/NACK & CQI, are time-division-multiplexed at LB level. In this case, assuming that the mobile stations UE1 to UE3 are present on the border of a cell in a power-limited multi-cell environment, the respective control resources for ACK/NACK and CQI of the mobile station UE3 are reduced in comparison with those for ACK/NACK and CQI of the other mobile stations UE1 and UE2. Accordingly, at least the properties of ACK/NACK & CQI of the mobile station UE3 are more degraded than the property of ACK/NACK of the mobile station UE1 and the property of CQI of the mobile station UE2.
As described above, if resources are allocated in a fixed amount to a plurality of control signals which differ from each other in amount of information, transmission frequency, required condition of quality, and the like, problems arise that required conditions are not satisfied because of a lack of the amount of a resource allocated, and that the efficiency of resource usage declines because of the allocation of an excessive amount of a resource. Particularly when resources are allocated to both of ACK/NACK and CQI in one TTI, fixing the ratio between resources for ACK/NACK and CQI leads to a problem.